A number of genes are being bred onto standard inbred mouse backgrounds, chiefly the NFS Swiss. These genes include ecotropic virus-inducing loci, virus-resistance genes, and linkage marker genes for chromosomal regions of particular interest. The V-loci congenics have permitted the initial demonstration that the virus-inducing loci were in fact viral genomes, have allowed characterization of the genetic control of their induction, have allowed mapping of some, have allowed biochemical identification of the proviruses from multi-copy high-virus strains, have allowed identification of the crucial importance of virus titer in early life for rate of subsequent leukemia and the general independence of leukemia experience on the source and chromosomal location of the endogenous provirus, have shown the crucial importance of maternal antibody in determining the life long viral phenotype, and have demonstrated the occurrence of germline reinsertions of viral genomes. Together, the congenics permit an unprecedented degree of genetic analysis and strain development for analysis of viral leukemogenesis.